Ultraviolet (UV) radiation is liable to cause skin cancer but it is the main source of vitamin D. Vitamin D photosynthesis takes place in skin at sub-erythemogenic UV doses, while larger exposures destroy vitamin D and increase DNA damage. Proper UV dosimetry is needed to obtain an optimal vitamin D status when skin cancer risk is minimal. A simple approach to such dosimetry using physically measured accumulated UV dose cannot provide a satisfactory quantification of vitamin D because of the complexity of the processes involved in vitamin D synthesis. A biological dosimeter of vitamin D synthetic UV radiation ('D-dosimeter') has been introduced earlier on the basis of an in vitro model of previtamin D photosynthesis. In the present study in vivo generation of 25-hydroxyvitamin D (25(OH)D) in serum of healthy volunteers exposed to UV radiation from the sunbed was accompanied by in vitro measurements of vitamin D formation using 'D-dosimeter'. It was found that the increase in serum 25(OH)D concentration depended both on the initial 25(OH)D level and on the cumulative sunbed exposure time. The observed linear correlation between in vivo and in vitro data can be used to estimate changes in vitamin D status after UV exposure using only one pre-exposure blood sample combined with further in vitro measurements.